A report by the Health Protection Agency into the outbreak of cryptosporidium has claimed bags of salad labelled “ready-to-eat” and sold by Morrisons and Asda were probably to blame. However, the retailers have contested the validity of the report’s methodology.

Three hundred people were reported to have contracted the infection in England and Scotland in May 2012. There were no fatalities, with most cases showing mild to moderate symptoms.

The agency found that out of a 25% sample of those who fell ill, 46% indicated that they had consumed the same mixed-salad product – a figure considered “extremely high” by the agency, leading them to their conclusions of culpability.

However, the Food Standards Agency (FSA), which was part of the outbreak control team led by the HPA, could not identify the source of the contamination in a separate investigation, it said in a statement. The FSA gathered information on the production and distribution of salad vegetables to try to identify the likely source of the outbreak. Probing the food chain including practice and procedures through each stage of growing, processing, packing and distribution of salad vegetables did not identify the origin, the FSA said.

“We very strongly dispute we were anything to with this… The HPA appears to be concerned with making an eye-catching announcement before being disbanded in two weeks’ time” - Morrisons

“We very strongly dispute we were anything to with this,” a spokesman for Morrisons said. “We didn’t see any complaints from any customers on this issue. Morrisons shares the same supply chain used for its bagged salads with several other retailers, meaning they would also have been affected by this outbreak. The HPA appears to be concerned with making an eye-catching announcement before being disbanded in two weeks’ time.”

From 1 April, the HPA will become part of Public Health England.

A spokeswoman for Asda said: “The HPA’s research is statistically flawed and does not prove that Asda is the source of the outbreak. Product safety is our top priority and if we had any serious concerns we’d act immediately. So far this hasn’t been necessary.”

Difficult to obtain evidence

Dr Clarence Tam, senior lecturer at the London School of Hygiene & Tropical Medicine, said: “Although the methodology appears sound, it is often difficult to obtain microbiological evidence of contamination in a specific food, because contamination is often transient or affects certain product batches, and it can be difficult to trace back to the exact product or supplier.

“This does not invalidate the method, though it makes it more difficult to establish definitively the source of infection. In this instance, there was strong statistical evidence for a particular type of pre-packed leaf vegetable. When the analysis looked at associations with specific retailers, there was statistical evidence only for certain retailers. However, the report is careful to mention that different retailers often share the same suppliers and that other retailers could also have been affected.”

The cryptosporidium parasite causes the disease cryptosporidiosis. The most common symptom is diarrhoea, which can range from mild to severe.  Potential causes include consumption of contaminated water or food, swimming in contaminated water, or contact with contaminated food or affected animals.

Dr Stephen Morton, regional director of the HPA’s Yorkshire and the Humber region and head of the multi-agency Outbreak Control Team, said: “It is often difficult to identify the source of short-lived outbreaks of this type as by the time that the outbreak can be investigated, the affected food and much of the microbiological evidence may no longer be available.”

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